1. Field of the Invention
The present invention relates to a process of selectively preparing a high-quality superconducting oxide thin film on the substrate of an electronic superconducting device.
2. Description of the Related Art
In recent years, various superconducting oxide materials such as yttrium (Y), bismuth (Bi) and thallium (Tl) compounds have been discovered which can exhibit superconductivity at their critical temperature which is higher than the temperature of liquid nitrogen. Studies are actively being undertaken to enable the application of thin films formed from these superconducting oxide materials to electronic superconducting devices. In order to provide good device properties, it is first necessary to prepare a high-quality superconducting oxide thin film having a smooth surface on a dielectric or semiconductor substrate.
The current methods of preparing the superconducting oxide thin film include sputtering, reactive co-evaporation, laser ablation, molecular beam epitaxy (MBE) and MOCVD. Among them, MBE is a film forming process which is superior in film thickness at atomic layer level and in controllability of crystalline structure. Such a process can prepare a thin film having an excellent evenness.
In order to realize the molecular beam state of evaporating matter, the MBE process requires the maintaining of a high vacuum growth chamber so that the mean free path of the evaporating matter will be maintained sufficiently. When an oxide thin film is to be produced by the MBE process, sufficient oxygen must be supplied to the thin film while maintaining a high vacuum. Since the oxygen molecules are less reactive, an increased amount of oxygen must be introduced into the growth chamber so as to supply the necessary amount of oxygen into the thin film. If the normal oxygen O.sub.2 is introduced into the growth chamber, the degree of vacuum in the growth chamber will be extremely deteriorated. Therefore active sources of oxygen such as ozone O.sub.3, nitrogen dioxide NO.sub.2 and the like are now used to oxidize the thin film more effectively with a minimum amount of oxygen which is introduced from out side of the growth chamber.
The effective oxidization of the thin film with ozone or nitrogen dioxide is due to the fact that the active oxygen source can be easily decomposed to form oxygen radicals (also called excited state, nascent state and atomic state oxygen) which are very active for oxidization (O.sub.3 .fwdarw.O*+O.sub.2, NO.sub.2 .fwdarw.O*+NO). On the contrary, the non-decomposed oxygen molecules are less active than the oxygen radicals.
However, O.sub.2 and NO produced from ozone or nitrogen dioxide simultaneously with the production of oxygen radicals will deteriorate the degree of vacuum in the growth chamber of the molecular beam epitaxy system since they remain in the chamber. In order to decompose ozone or nitrogen dioxide, it is further necessary to obtain thermal energy from the substrate surface. Additionally, the ozone and nitrogen dioxide can be reactive only after they have diffused to some extent on the surface of the substrate.